Telecommunications systems include customer premise equipment (CPE), local loops connecting each customer premises to a central office or other node, the nodes providing switching and signaling for the system, and internode trunks connecting the various nodes. The customer premise equipment (CPE) includes telephones, modems for communicating data over phone lines, and computer and other devices that can directly communicate video, audio, and other data over a data link. The network nodes include traditional circuit-switched nodes that have transmission paths dedicated to specific users for the duration of a call and employ continuous, fixed-bandwidth transmission and packet-switch nodes that allow dynamic bandwidths, dependent on the application. The transmission media between nodes may be wireline or wireless.
One type of wireline transmission media is optical fiber that has a thin strand of glass that is designed to carry information using pulses of light. Separate optical fibers are bundled together and encased in an outer sheath to form fiber cables. Optical fiber provides users with higher reliability, superior performance, and greater flexibility than traditional copper-based systems.
Optical transmission facilities are installed in the form of synchronous optical network (SONET) rings. SONET defines a line rate hierarchy and frame format as described by the American National Standards Institute (ANSI) T1.105 and T1.106 specifications. Typically, nodes on a SONET ring provide add-drop multiplexing for traffic on the ring and may provide limited switching functionality. SONET rings are typically bi-directional to provide redundant transmission paths and protection in case of a line or node failure.
Typically, SONET network elements are connected to “digital cross-connects” (DCSs) that provide switching functionality for traffic on the ring and for traffic between networks. DCSs, which are typically expensive, also provide line testing capability through fixed test ports that are accessible by a test head. Test heads can passively monitor connections, or use “split” lines and intrusively test and troubleshoot a circuit by both transmitting signals to the circuit and monitoring signals received from it.